Current analog fiber-optic links using intensity modulators are used in both defense, aerospace and commercial applications for transmission of analog radio frequency (RF) signals from antennas. The current fiber-optic links have limited performance, particularly at high RF frequencies (10 to >100 GHz) due to limitations of modulators declining conversion efficiency at higher RF frequencies, and small-area high frequency photodetector with limited optical power handling before non-linear saturation and damage occur. These limitations posed by the modulators and photodetectors severely limit the performance and implementation viability of the conventional RF fiber optic links for these high-RF frequency signal transmission.
This invention is an extension of the invention by the same inventor, described in U.S. Pat. No. 7,660,491, by Suwat Thaniyavarn, issued to EOSpace Inc. on Feb. 9, 2010).
Current analog RF fiber-optic links for higher RF frequency signals have problems to be solved. There are currently no viable RF coax or waveguide transmission lines for transmitting RF signals with extremely high frequencies (10 to >100 GHz) from an antenna, due to high loss, weight and limited bandwidth. Instead, these high frequency RF signals are processed or down-converted near the antenna, resulting in significant loss of fidelity and limited-bandwidth of channelizing techniques.
An analog RF fiber-optic link offers potential for enormous bandwidth, transmission-length independent loss and wideband RF-photonic signal processing. However, current fiber-optic link performance must be significantly improved, particularly for very-high mm-wave frequencies. These problems are due to the fact that although optical modulators (used for converting RF electrical signals into optical signals for signal transmission via optical fibers) and photodetectors (used to convert the transmitted optical signals via optical fibers back to RF signals at the receivers) can be made to operate at extremely high RF frequencies, they are less efficient at higher RF frequencies. In addition, photodetectors capable of very high RF frequency operation are typically very small in size, which is necessary for higher frequency operation, and therefore are only capable of very limited optical power handling. This limited optical power handling issue with high frequency photodetectors means higher-optical power source cannot be used. Limiting optical power at the photodetector means limited RF fiber-optic link performance in terms of poor link loss, high noise figure and lower dynamic range. These issues limit the current implementation of fiber-optic transmission of RF signals at very high RF (20->100 GHz) frequencies.